1. Field of the Invention
The present invention relates to a reflecting apparatus and, more particularly, to a reflecting apparatus, which constitutes a part of a projection optical system of an exposure apparatus.
2. Description of the Related Art
Conventionally, in photolithography for manufacturing, e.g., a semiconductor element, a pattern formed on a mask (or reticle) is projected and transferred by exposure onto a photosensitive substrate, such as a wafer, via a projection optical system. In this projection exposure, a projection exposure apparatus, such as a step and repeat exposure apparatus (a so-called stepper) or a step and scan exposure apparatus (a so-called scanner) is used.
An apparatus of this conventional type utilizes exposure light, such as a g-line (wavelength: 436 nm) or an i-line (wavelength-365 nm). These days, the apparatus often adopts, e.g., KrF excimer laser light (wavelength: 248 nm) or ArF excimer laser light (wavelength: 193 nm).
A catadioptric optical system is available as a projection optical system for projection exposure with exposure light having the above-described wavelength ranges. The catadioptric optical system can obtain a reduction ratio high enough for mask pattern projection and can downsize the projection optical system itself.
Unfortunately, even such a catadioptric optical system generates aberration due to, e.g., a manufacturing error or an assembly error of optical elements, which form the optical system, or thermal deformation of an optical element, which is caused upon partially absorbing exposure light. This aberration degrades the quality of the pattern image projected onto the photosensitive substrate.
In view of this, there is proposed an apparatus, which comprises various kinds of reflecting optical elements, e.g., reflecting mirrors, such as a plane mirror and a concave mirror, and changes the shape of the surface (reflection surface) of the mirror to correct the aberration of the optical system.
FIG. 16 shows a reflecting apparatus described in Japanese Patent Laid-Open No. 2004-64076 as an example of a reflecting apparatus whose surface contour is changeable.
This reflecting apparatus comprises a mirror 20 and a low-rigidity actuator 30. The low-rigidity actuator 30 locally deforms the rear surface (the lower surface in FIG. 16) of the mirror 20 at a predetermined position. The low-rigidity actuator 30 utilizes, e.g., a voice coil motor, an air-pressure actuator, or an EI core actuator.
In the reflecting apparatus, which deforms the surface contour of the mirror, the actuator used for deformation generates heat. This often results in unintended deformation of the mirror.
If one increases the magnitude of a force relative to the heat amount in consideration of the heat generation, the movable element of the actuator may become larger and more complicated. In this case, disturbing vibration from, e.g., the floor, often vibrates the mirror.
If one circulates a refrigerant by arranging a refrigerant pipe to cool the movable element (which acts as a heat generation portion) of the actuator in consideration of the heat generation, vibration from the refrigerant pipe vibrates the mirror.